The present invention relates to the control of BLDC motors and in particular to the control of sinusoidally driven BLDC motors.
Driving BLDC motors with sinusoidal currents improves efficiency of operation and reduces acoustic noise when compared with 120° block mode commutation. As with all schemes for driving BLDC motors, efficient implementation requires that the rotor position and the difference between the applied voltage and the resulting current are monitored such that sinusoidal currents may be applied in phase with the rotor rotation. Present methods for monitoring motor position in sinusoidally driven BLDC motors add significantly to the overall system complexity and cost.
An example of the above is the monitoring scheme disclosed in the Microchip Technology Inc. Application Note AN1078; “Sensorless Field Oriented Control of PMSM Motors.” This discloses a sensorless field oriented control (FOC) method of monitoring the rotor position. This method, for a 3 phase motor, uses at least 2 shunt resistors to measure the phase currents in two of the motor phases and then calculates the third current. The phase currents are then converted to a two axis vector system, which is rotated through a calculated transformation angle (based on a previous iteration of the control loop). The rotation generates two new current variables from which a corresponding pair of voltage variables is generated. A new transformation angle is estimated, and the voltage variables are rotated back to a stationary reference frame and converted to 3 phase voltage values. The 3 phase voltage values can then be used to calculate suitable driving voltages. As the FOC algorithm is very complex mathematically, this method requires a very high calculation power DSP (digital signal processor). Additionally, this method requires at least 2 shunt resistors to measure the motor phase currents needed as input for the FOC algorithm. Both add to the cost and complexity involved with implementing such a system.
The document U.S. Pat. No. 7,294,982 discloses measurement of the current zero crossing points during steady state (constant speed, constant load) operation to control a sinusoidally driven BLDC motor. As this method is not reliable during speed or load changes or during driving in PWM mode, a second control scheme is utilised. This involves interrupting the driving current to measure the BEMF and controlling the driving voltage so that a measured motor drive current coincides with calculated current instruction signal. To achieve this, target current values (SPN CRNT DATA) are calculated and subsequently processed in a current error detector, a current control filter and a profile generator module. Calculating target current values in addition to PWM driving voltages at least doubles the computational complexity of the algorithm with respect to calculating PWM driving voltages only. The algorithm therefore also requires expensive hardware such as a DSP for implementation.
The document U.S. Pat. No. 7,525,268 discloses a method of measuring the current zero crossing for deducing the distance between the time instant of the expected zero crossing and the actual zero crossing instant; the further means is suitable for determining the phase shift between the driving voltage of the electric motor and the induced back electromotive force based on the distance. Typically it is desired that the BEMF zero crossing is aligned with the current zero crossing for optimum motor efficiency. Therefore this method can not be used together with the method of estimating the rotor position based on back EMF zero crossings.